Multicode direct sequence spread spectrum

ABSTRACT

In this patent, we present MultiCode Direct Sequence Spread Spectrum (MC-DSSS) which is a modulation scheme that assigns up to N DSSS codes to an individual user where N is the number of chips per DSSS code. When viewed as DSSS, MC-DSSS requires up to N correlators (or equivalently up to N Matched Filters) at the receiver with a complexity of the order of N 2  operations. In addition, a non ideal communication channel can cause InterCode Interference (ICI), i.e., interference between the N DSSS codes. In this patent, we introduce new DSSS codes, which we refer to as the “MC” codes. Such codes allow the information in a MC-DSSS signal to be decoded in a sequence of low complexity parallel operations which reduce the ICI. In addition to low complexity decoding and reduced ICI. MC-DSSS using the MC codes has the following advantages: (1) it does not require the stringent synchronization DSSS requires, (2) it does not require the stringent carrier recovery DSSS requires and (3) it is spectrally efficient.

This application is a REISSUE of Ser. No. 08/186,784 filed Jan. 24, 1994is a continuation-in-part of U.S. application Ser. No. 07/861,725 filedMar. 31, 1992, now U.S. Pat. No. 5,282,222, the benefit of the filingdate of which is hereby claimed under 35 U.S.C. §120.

FIELD OF THE INVENTION

The invention deals with the field of multiple access communicationsusing Spread Spectrum modulation. Multiple access can be classified aseither random access, polling, TDMA, FDMA, CDMA or any combinationthereof. Spread Spectrum can be classified as Direct Sequence,Frequency-Hopping or a combination of the two.

BACKGROUND OF THE INVENTION

Commonly used spread spectrum techniques are Direct Sequence SpreadSpectrum (DSSS) and Code Division Multiple Access (CDMA) as explained inChapter 8 of “Digital Communication” by J. G. Proakis, Second Edition,1991, McGraw Hill, DSSS is a communication scheme in which informationbits are spread over code bits (generally called chips). It is customaryto use noise-like codes called pseudo random noise (PN) sequences. ThesePN sequences have the property that their auto-correlation is almost adelta function and their cross-correlation with other codes is almostnull. The advantages of this information spreading are:

1. The transmitted signal can be buried in noise and thus has a lowprobability of intercept.

2. The receiver can recover the signal from interferers (such as othertransmitted codes) with a jamming margin that is proportional to thespreading code length.

3. DSSS codes of duration longer than the delay spread of thepropagation channel can lead to multipath diversity implementable usinga Rake receiver.

4. The FCC and the DOC have allowed the use of unlicensed low power DSSSsystems of code lengths greater than or equal to 10 in some frequencybands (the ISM bands).

It is the last advantage (i.e., advantage 4. above) that has given muchinterest recently to DSSS.

An obvious limitation of DSSS systems is the limited throughput they canoffer. In any given bandwidth, B, a code of length N will reduce theeffective bandwidth to B/N. To increase the overall bandwidthefficiency, system designers introduced Code Division Multiple Access(CDMA) where multiple DSSS communication links can be establishedsimultaneously over the same frequency band provided each link uses aunique code that is noise-like. CDMA problems are:

1. The near-far problem: a transmitter “near” the receiver sending adifferent code than the receiver's desired code produces in the receivera signal comparable with that of a “far” transmitter sending the desiredcode.

2. Synchronization of the receiver and the transmitter is complex(especially) if the receiver does not know in advance which code isbeing transmitted.

SUMMARY OF THE INVENTION

We have recognized that low power DSSS systems complying with the FCCand the DOC regulations for the ISM bands would be ideal communicatorsprovided the problems of CDMA could be resolved and the throughput couldbe enhanced. To enhance the throughput, we allow a single link (i.e., asingle transceiver) to use more than one code at the same time. To avoidthe near-far problem only one transceiver transmits at a time. In thispatent, we present Multi-Code Direct Sequence Spread Spectrum (MC-DSSS)which is a modulation scheme that assigns up to N codes to an individualtransceiver where N is the number of chips per DSSS code. When viewed asDSSS, MC-DSSS requires up to N correlators (or equivalently up to NMatched Filters) at the receiver with a complexity of the order of N²operations. When N is large, this complexity is prohibitive. Inaddition, a nonideal communication channel can cause InterCodeInterference (ICI), i.e., interference between the N DSSS codes at thereceiver. In this patent, we introduce new codes, which we refer to as“MC” codes. Such codes allow the information in a MC-DSSS signal to bedecoded in a sequence of low complexity parallel operations whilereducing the ICI. In addition to low complexity decoding and ICIreduction, our implementation of MC-DSSS using the MC codes has thefollowing advantages:

1. It does not require the stringent synchronization DSSS requires.Conventional DSSS systems requires synchronization to within a fractionof a chip whereas MC-DSSS using the MC codes requires synchronization towithin two chips.

2. It does not require the stringent carrier recovery DSSS requires.Conventional DSSS requires the carrier at the receiver to be phaselocked to the received signal whereas MC-DSSS using the MC codes doesnot require phase locking the carriers. Commercially available crystalshave sufficient stability for MC-DSSS.

3. It is spectrally efficient.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic showing for the Baseband Transmitter for the xthMC-DSSS frame: d(k)=[d(1,x) d(2,x) . . . d(N,k)] where c(i)=[c(1,i)c(2,i)] is the ith code and Sym(k)=[sym(1,k) sym(N,k)] is the kthinformation-bearing vector containing N symbols.

FIG. 2 is a schematic showing a Baseband Receiver for the kth receivedMC-DSSS frame: d′(k)=[d′(1,k) d′(2,k) . . . d′(N,k)] where c(i)=[c(1,i)c(2,i) . . . c(N,i)] is the ith code, Sy{circumflex over(m)}(k)=[sy{circumflex over (m)}(1,k) sy{circumflex over (m)}(2,k) . . .sy{circumflex over (m)}(N,k)] is the estimate of the Kthinformation-bearing vector Sym(k) and

FIG. 3 is a schematic showing of the ith MC code c(i)=[c(i,1) c(i,2) . .. c(i,NO) where i can take one of the N values: 1,2, . . . Ncorresponding to the position of the single ‘1’ at the input of thefirst N-point transform.

FIG. 4 is a schematic showing the alternate transmitter for the kthMC-DSSS frame: d(k)=[d(1,k), d(2,k) . . . d(N,k)] using the MC codesgenerated in FIG. 3 where Sym(k)=[Sym(1,k)Sym(2k) . . . Sym(N,k)] is thekth information-bearing vector contacting N symbols.

FIG. 5 is the alternate receiver for the kth received MC-DSSS framed′(k)=[d′(1k)d′(2,K) . . .d′(N,k)] using MC codes generated in FIG. 3where Sy{circumflex over (m)}(k)=[sy{circumflex over (m)}(1,k)sy{circumflex over (m)}(2k) . . . sy{circumflex over (m)}(N,k)] is theestimate of the information-bearing vetor Sym(k).

FIG. 6 is a schematic showing the Baseband Transmitter of the kth DataFrame X(k) where Sym(N)=[sym(1,k) sym(2,k) . . . sym(N,k)] is the kthinformation-bearing vector d(k)=[c(1,k) d(2,k) . . . d(N,k)] is the kthMC-DSSS frame v(k)=[v(1,k) v(2,k) . . . v((1+β)MN,k)], βε(0,1), M=1,2,3. . . and X(k)=[x(1k) x(2,k)], Z=Z=1, 2, 3, . . . .

FIG. 7 is a schematic showing the Baseband Receiver for the kth receivedData Frame X′(k) where Sy{circumflex over (m)}(N)=[sy{circumflex over(m)}(1,k)] sy{circumflex over (m)}(2,k) . . . sy{circumflex over(m)}(N,k)] is the estimate of the kth information-bearing vectord′(k)=[d′(1,k) d′(2k) . . . d′(N,k)] is the kth received MC-DSSS framev′(k)=[v′(1,k) v(2k) . . . v′((1+β) MN,k)], Bε(0,1), M=1,2,3, . . . andX′(k)=[x′(1,k) x′(2,k) . . . r′(Z,k)], Z=1,2,3 . . . .

FIG. 8 is a schematic showing the Randomizer Transform (RT) where a (1)a (2) . . . a (N) are complex constants chosen randomly.

FIG. 9 is a schematic showing the Permutation Transform (PT).

FIG. 10 is a schematic showing (a) the shaping of a MC-DSSS frame and(b) the unshaping of a MC-DSSS frame where d(k)=[d(1,k) d(2,k) . . .d(N,k)] is the kth MC-DSSS frame g(k)=[g(1,k) g(2k) . . . g(MN,k)],M=1,2,3, . . . , v(k)=[v(1,k) v(2,k) . . . v((1+β) MN,k)], Bε(0,1)d′(k)=[d(1,k) d(2,k) . . . d(N,K)] is the kth received MC-DSSS frameg′(k)=[g′(1,k) g′(2,k) . . . g′(M′N,k)] and v′(k)=[v(1,k) v′(2,k) . . .v′((1+β) M′N,k)], M′=1,2,3, . . . .

FIG. 11 is a schematic showing (a) Description of the alias/windowoperation (b) Description of dealias/dewindow operation, where 1/T isthe symbol rate.

FIG. 12 is a schematic showing the frame structure for data transmissionfrom source (Node A) to destination (Node B).

FIG. 13 is a schematic showing the baseband transmitter for one requestframe v where c=[c(1) c(2) . . . c(1)] is the DSSS code, v=[v(1) v(2) .. . v((1+β)MI)], βε(0,1), M=1,2, . . . and I is the length of the DSSScode.

FIG. 14 is a schematic showing the baseband receiver for the receivedrequest frame where c=[c(1) c(2) . . . c(1)] is the DSSS code for therequest frame, d′=[d(1) d(2) . . . d(1)] is the received request frame,v′=[v′(1) v′((1+β) MI)], βε(0,1), M=1,2, . . . and l is the length ofthe DSSS code.

FIG. 15 is a schematic showing the baseband transmitter for one addressframe where c=[c(1) c(2) . . . c(1)] is the CDMA code for the addressframe, v=[v(1) v(2) . . . v(1+β) MI)], βε(0,1), M=1,2, . . . and l′ isthe length of the CDMA code.

FIG. 16 is a schematic showing the baseband receiver the address wherec=[c(1) c(2) . . . c(I′)] is the CDMA code for the address frame,d′=[d(1) d(2) . . . d(I)] is the received address frame, v′[v′(1) v′(2). . . v′((1+β) MI′)], βε(0,1), M=1,2, . . . and I′ is the length of theCDMA code.

FIG. 17 is a schematic showing the baseband transmitter for Ack. Framewhere c=[c(1) c(2) . . . c(I′)] is the DSSS code for the Ack. frame,v=[v(1) v(2) . . . v((I+β) MI′)] βε(0,1), M=1,2,3, . . . and I′ is thelength of the DSSS code.

FIG. 18 is a schematic showing the baseband receiver for the ack. framewhere c=[c(1) c(2) . . . c(I″)] is the DSSS code for the Ack. frame,d′=[d(1) d(2) . . . d′(I″)] is the received Ack. frame, v′=[v′(1) v(2) .. . v′(1+β) MI″)], βε(0,1), M=1,2, . . . and I″ is the length of theDSSS code.

FIG. 19 is a schematic showing the passband transmitter for a packetwhere f_(o) is the IF frequency and f_(o)+f_(c) is the RF frequency.

FIG. 20 is a schematic showing the passband receiver for a packet wheref_(o) is the IF frequency and f_(o)+f_(c) is the RF frequency.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 illustrates the transmitter of the MC-DSSS modulation techniquegenerating the kth MC-DSSS frame bearing N symbols of information. Thesymbols can be either analog or digital.

A converter 10 converts a stream of data symbols into plural sets of Ndata symbols each. A computing means 12 operates on the plural sets of Ndata symbols to produce modulated data symbols corresponding to aninvertible randomized spreading of the stream of data symbols. Acombiner 14 combines the modulated data symbols for transmission. Thecomputing means shown in FIG. 1 includes a source 16 of N directsequence spread spectrum code symbols and a modulator 18 to modulateeach ith data symbol from each set of N data symbols with the I codesymbol from the N code symbol to generate N modulated data symbols, andthereby spread each I data symbol over a separate code symbol.

FIG. 2 illustrates the receiver of the MC-DSSS modulation techniquesaccepting the kth MC-DSSS frame and generating estimates for thecorresponding N symbols of information. The dot product in FIG. 2 can beimplemented as a correlator. The detector can make either hard decisionsor soft decisions.

A sequence of modulated data symbols is received at 22 in which thesequence of modulated data symbols has been generated by the transmittersuch as is shown in FIG. 1 or 4. A second computing means 24 operates onthe sequence of modulated data symbols to produce an estimate of thesecond string of data symbols. The computing means 24 shown in FIG. 2includes a correlator 26 for correlating each I modulated data symbolfrom the received sequence of modulated data symbols with the I codesymbol from the set of N code symbols and a detector 28 for detecting anestimate of the data symbols from output of the correlator 26.

FIG. 3 illustrates the code generator of the MC codes. Any one of the PN-point transforms in FIG. 3 consists of a reversible transform to theextent of the available arithmetic precision. In other words, withfinite precision arithmetic, the transforms are allowed to add a limitedamount of irreversible error.

One can use the MC-DSSS transmitter in FIG. 1 and the MC-DSSS receiverin FIG. 2 together with the MC codes generated using the code generatorin FIG. 3 in order to implement MC-DSSS using the MC codes.

An alternative transmitter to the one in FIG. 1 using the MC codes inFIG. 3 is shown in FIG. 4.

The alternative transmitter shown in FIG. 4 includes a transformer 20for operating on each set of N data symbols to generate N modulated datasymbols as output. A series of transforms are shown.

An alternative receiver to the one in FIG. 2 using the MC codes in FIG.3 is shown in FIG. 5. L pilots are required in FIG. 5 for equalization.

Both transmitters in FIGS. 1 and 4 allow using shaper 30 in diversitymodule 32 shaping and time diversity of the MC-DSSS signal as shown inFIG. 6. We will refer to the MC-DSSS frame with shaping and timediversity as a Data frame.

Both receivers in FIGS. 2 and 5 allow diversity combining followed bythe unshaping of the Data frame as shown in FIG. 7. A Synch. is requiredin FIG. 7 for frame synchronization.

In addition to the Data frames, we need to transmit (1) all of the Lpilots used in FIG. 5 to estimate and equalize for the various types ofchannel distortions, (2) the Synch. signal used in FIG. 7 for framesynchronization, and (3) depending on the access technique employed, thesource address, destination address and number of Data frames. We willrefer to the combination of all transmitted frames as a packet.

PREFERRED EMBODIMENTS OF THE INVENTION

Examples of the N-point transforms in FIG. 3 are a Discrete FourierTransform (DFT), a Fast Fourier Transform (FFT), a Walsh Transform (WT),a Hilbert Transform (HT), a Randomizer Transform (RT) as the oneillustrated in FIG. 8, a Permutator Transform (PT) as the oneillustrated in FIG. 9, an Inverse DFT (IDFT), an Inverse FFT (IFFT), anInverse WT (IWT), an Inverse HT (IHT), an Inverse RT (IRT), an InversePT (IPT), and any other reversible transform. When L=2 with the firstN-point transform being a DFT and the second being a RT, we have asystem identical to the patent: “Method and Apparatus for MultipleAccess between Transceivers in Wireless Communications using OFDM SpreadSpectrum” by M. Fattouche and H. Zaghloul, filed in the U.S. Pat Officein Mar. 31, 1992, Ser. No. 07/861,725.

Preferred shaping in FIG. 6 consists of an Mth order interpolationfilter followed by an alias/window operation as shown in FIG. 10a. TheAlias/window operation is described in FIG. 11a where a raised-cosinepulse of rolloff β is applied. The interpolation filter in FIG. 10a canbe implemented as an FIR filter or as an NM-point IDFT where the firstN(M−1)/2 points and the last N(M−1)/2 points at the input of the IDFTare zero. Preferred values of M are 1,2,3 and 4.

Preferred unshaping in FIG. 7 consists of a dealias/dewindow operationfollowed by a decimation filter as shown in FIG. 10b. Thedealias/dewindow operation is described in FIG. 11b.

Time Diversity in FIG. 6 can consist of repeating the MC-DSSS frameseveral times. It can also consist of repeating the frame several timesthen complex conjugating some of the replicas, or shifting some of thereplicas in the frequency domain in a cyclic manner.

Diversity combining in FIG. 7 can consist of cophasing, selectivecombining, Maximal Ratio combining or equal gain combining.

In FIG. 5, L pilots are used to equalize the effects of the channel oneach information-bearing data frame. The pilot frames can consist ofData frames of known information symbols to be sent either before,during or after the data, or of a number of samples of known valuesinserted within two transformations in FIG. 4. A preferred embodiment ofthe pilots is to have the first pilot consisting of a number of framesof known information symbols. The remaining pilots can consist of anumber of known information symbols between two transforms. The Lestimators can consist of averaging of the pilots followed by either aparametric estimation or a nonparametric one similar to the channelestimator in the patent: “Method and Apparatus for Multiple Accessbetween Transceivers in Wireless Communications using OFDM SpreadSpectrum” by M. Fattouche and H. Zaghloul, filed in the U.S. Pat Officein Mar. 31, 1992, Ser. No. 07/861,725.

When Node A intends to transmit information to Node B, a preferredembodiment of a packet is illustrated in FIG. 12: a Request frame 40, anAddress frame, an Ack. frame, a Pilot frame 36 and a number of Dataframes 38. The Request frame is used (1) as a wake-up call for all thereceivers in the band, (2) for frame synchronization and (3) for packetsynchronization. It can consist of a DSSS signal using one PN coderepeated a number of times and ending with the same PN code with anegative polarity. FIGS. 13 and 14 illustrate the transmitter and thereceiver for the Request frame respectively. In FIG. 14, the dot productoperation can be implemented as a correlator with either hard or softdecision (or equivalently as a filter matched to the PN code followed bya sample/hold circuit). The Request frame receiver is constantlygenerating a signal out of the correlator. When the signal is above acertain threshold using the level detector, (1) a wake-up call signal isconveyed to the portion of the receiver responsible for the Addressframe and (2) the frames are synchronized to the wake-up call. Thepacket is then synchronized to the negative differential correlationbetween the last two PN codes in the Request frame using a decoder asshown in FIG. 14.

The Address frame can consist of a CDMA signal where one out of a numberof codes is used at a time. The code consists of a number of chips thatindicate the destination address, the source address and/or the numberof Data frames. FIGS. 15 and 16 illustrate the transmitter and thereceiver for the Address frame respectively. Each receiverdifferentially detects the received Address frame, then correlates theoutcome with it is own code. If the output of the correlator is above acertain threshold, the receiver instructs its transmitter to transmit anAck. Otherwise, the receiver returns to its initial (idle) state.

The Ack. frame is a PN code reflecting the status of the receiver, i.e.whether it is busy or idle. When it is busy, Node A aborts itstransmission and retries some time later. When it is idle, Node Aproceeds with transmitting the Pilot frame and the Data frames. FIGS. 17and 18 illustrate the transmitter and the receiver for the Address framerespectively.

An extension to the MC-DSSS modulation technique consists of passbandmodulation where the packet is up-converted from baseband to RF in thetransmitter and later down-converted from RF to baseband in thereceiver. Passband modulation can be implemented using IF sampling whichconsists of implementing quadrature modulation/demodulation in anintermediate Frequency between baseband and RF, digitally as shown inFIGS. 19 and 20 which illustrate the transmitter and the receiverrespectively. IF sampling trades complexity of the analog RF components(at either the transmitter, the receiver or both) with complexity of thedigital components. Furthermore, in passband systems carrierfeed-through is often a problem implying that the transmitter has toensure a zero dc component. Such a component reduces the usablebandwidth of the channel. In IF sampling the usable band of the channeldoes not include dc and therefore is the dc component is not a concern.

A further extension to the MC-DSSS modulation technique consists ofusing antenna Diversity in order to improve the Signal-to-Ratio level atthe receiver. A preferred combining technique is maximal selectioncombining based on the level of the Request frame at the receiver.

We claim:
 1. A transceiver for transmitting a first stream of datasymbols, the transceiver comprising: a converter for converting thefirst stream of data symbols into plural sets of N data symbols each;first computing means for operating on the plural sets of N data symbolsto produce modulated data symbols corresponding to an invertiblerandomized spreading of the first stream of data symbols; and means tocombine the modulated data symbols for transmission.
 2. The transceiverof claim 1 in which the first computing means includes comprises: asource of N more than one and up to M direct sequence spread spectrumcode symbols codes, where M is the number of chips per direct sequencespread spectrum code; and a modulator to modulate each ith data symbolfrom each set of N data symbols with the ith a code symbol from the Ncode symbol up to M direct sequence spread spectrum codes to generate Nmodulated data symbols, and thereby spread each ith data symbol set ofdata symbols over a separate code symbol .
 3. The transceiver of claim 2in which the code symbols direct sequence spread spectrum codes aregenerated by operation of a non-trivial N point transform on a sequenceof input signals.
 4. The transceiver of claim 1 in which the firstcomputing means includes comprises: a transformer for operating on eachset of N data symbols to generate N modulated data symbols as output,the N modulated data symbols corresponding to spreading of each ith datasymbol over a separate code symbol selected from a set of more than oneand up to M codes, where M is the number of chips per code; and means tocombine the modulated data symbols for transmission.
 5. The transceiverof claim 4 in which the transformer effectively applies a firsttransform selected from the group comprising consisting of a Fouriertransform and a Walsh transform to the N data symbols.
 6. Thetransceiver of claim 5 in which the first transform is a Fouriertransform and it is followed by a randomizing transform.
 7. Thetransceiver of claim 6 in which the first transform is a Fouriertransform and it is followed by a randomizing transform and a secondtransform selected from the group comprising consisting of a Fouriertransform and a Walsh transform.
 8. The transceiver of claim 4 in whichthe transformer effectively applies a first inverse transform selectedfrom the group comprising consisting of a randomizer transform, aFourier transform and a Walsh transform to the N data symbols, followedby a first equalizer and a second inverse transform selected from thegroup comprising consisting of a Fourier transform and a Walshtransform.
 9. The transceiver of claim 8 in which the second transformis followed by a second equalizer.
 10. The transceiver of claim 1further including comprising: means for receiving a sequence ofmodulated data symbols, the modulated data symbols having been generatedby invertible randomized spreading of a second stream of data symbols;and second computing means for operating on the sequence of modulateddata symbols to produce an estimate of the second stream of datasymbols.
 11. The transceiver of claim 10 further including comprisingmeans to apply diversity to the modulated data symbols beforetransmission, and means to combine received diversity signals.
 12. Thetransceiver of claim 10 in which the second computing means includescomprises: a correlator for correlating each ith modulated data symbolfrom the received sequence of modulated data symbols with the ith codesymbol a code from the a set of N code symbols more than one and up to Mcodes, where M is the number of chips per code; and a detector fordetecting an estimate of the data symbols from output of the correlator.13. The transceiver of claim 10 in which the second computing meansincludes comprises an inverse transformer for regenerating an estimateof the N data symbols.
 14. The transceiver of claim 1 further includingcomprising a shaper for shaping the combined modulated data symbols fortransmission.
 15. The transceiver of claim 1 further includingcomprising means to apply diversity to the combined modulated datasymbols before transmission.
 16. The transceiver of claim 1 in which theN data symbols include a pilot frame and a number of data frames, and ispreceded by a request frame, wherein the request frame is used to wakeup receiving transceivers, synchronize reception of the N data symbolsand convey protocol information.
 17. A transceiver for transmitting afirst stream of data symbols and receiving a second stream of datasymbols, the transceiver comprising: a converter for converting thefirst stream of data symbols into plural sets of N data symbols each;first computing means for operating on the plural sets of N data symbolsto produce sets of N modulated data symbols corresponding to aninvertible randomized spreading of each set of N data symbols over Ncode symbols more than one and up to M direct sequence spread spectrumcodes; means to combine the modulated data symbols for transmission;means for receiving a sequence of modulated data symbols, the modulateddata symbols having been generated by an invertible randomized spreadingof a second stream of data symbols over N code symbols more than one andup to M direct sequence spread spectrum codes; second computing meansfor operating on the sequence of modulated data symbols to produce anestimate of the second stream of data symbols; and means to combineoutput from the second computing means.
 18. The transceiver of claim 17in which the first computing means includes comprises: a source of N thedirect sequence spread spectrum code symbols codes; and a modulator tomodulate each ith data symbol from each set of N data symbols with theith code symbol a code from the N code symbol up to M direct sequencespread spectrum codes to generate N modulated data symbols, and therebyspread each ith data symbol over a separate direct sequence spreadspectrum code symbol .
 19. The transceiver of claim 18 in which the codesymbols direct sequence spread spectrum codes are generated by operationof plural non-trivial N point transforms on a random sequence of inputsignals.
 20. The transceiver of claim 17 in which the first computingmeans includes comprises: a transformer for operating on each set of Ndata symbols to generate N modulated data symbols as output, the Nmodulated data symbols corresponding to spreading of each ith datasymbol over a separate code symbol .
 21. The transceiver of claim 17 inwhich the second computing means includes comprises: a correlator forcorrelating each ith modulated data symbol from the received sequence ofmodulated data symbols with the ith code symbol a code from the set of Ncode symbols up to M direct sequence spread spectrum codes; and adetector for detecting an estimate of the data symbols from the outputof the correlator.
 22. The transceiver of claim 17 in which the secondcomputing means includes comprises an inverse transformer forregenerating an estimate of the N data symbols.
 23. A method ofexchanging data streams between a plurality of transceivers, the methodcomprising the steps of: converting a first stream of data symbols intoplural sets of N data symbols each; operating on the plural sets of Ndata symbols to produce modulated data symbols corresponding to aspreading of the first stream of data symbols over N code symbols morethan one and up to M direct sequence spread spectrum codes; combiningthe modulated data symbols for transmission; and transmitting themodulated data symbols from a first transceiver at a time when no otherof the plurality of transceivers is transmitting.
 24. The method ofclaim 23 in which the spreading is an invertible randomized spreadingand operating on the plural sets of N data symbols includes comprisesmodulating each ith data symbol from each set of N data symbols with theith code symbol a code from the N code symbols up to M direct sequencespread spectrum codes to generate N modulated data symbols, and therebyspread each ith data symbol over a separate code symbol .
 25. The methodof claim 23 in which the spreading is an invertible randomized spreadingand operating on the plural sets of N data symbols includes comprises:transforming, by application of a transform, each set of N data symbolsto generate N modulated data symbols as output.
 26. The method of claim25 in which transforming each set of N data symbols includes comprisesapplying to each set of N data symbols a randomizing transform and atransform selected from the group comprising consisting of a Fouriertransform and a Walsh transform.
 27. The method of claim 25 in whichtransforming each set of N data symbols includes comprises applying toeach set of N data symbols a Fourier transform, a randomizing transformand a transform selected from the group comprising consisting of aFourier transform and a Walsh transform.
 28. The method of claim 25 inwhich transforming each set of N data symbols includes comprisesapplying to each set of N data symbols a first transform selected fromthe group comprising consisting of a Fourier transform and a Walshtransform, a randomizing transform and a second transform selected fromthe group comprising consisting of a Fourier transform and a Walshtransform.
 29. The method of claim 23 further including comprising thestep of: receiving, at a transceiver distinct from the firsttransceiver, the sequence of modulated data symbols; and operating onthe sequence of modulated data symbols to produce an estimate of thefirst stream of data symbols.
 30. The method of claim 29 in whichoperating on the sequence of modulated data symbols includes comprisesthe steps of: correlating each ith modulated data symbol from thereceived sequence of modulated data symbols with the ith code symbolfrom the set of N code symbols a code from the up to M direct sequencespread spectrum codes; and detecting an estimate of the first stream ofdata symbols from output of the correlator.
 31. The method of claim 23further including comprising the step of shaping the modulated datasymbols before transmission.
 32. The method of claim 23 furtherincluding comprising the step of applying diversity to the modulateddata symbols before transmission.
 33. A transceiver for transmitting afirst stream of data symbols, the transceiver comprising: a converterfor converting the first stream of data symbols into plural sets of datasymbols each; first computing means for operating on the plural sets ofdata symbols to produce modulated data symbols corresponding to aninvertible randomized spreading of the first stream of data symbols overmore than one and up to M direct sequence spread spectrum codes, whereeach direct sequence spread spectrum code has M chips; and means tocombine the modulated data symbols for transmission.
 34. The transceiverof claim 33 further comprising: means for receiving a sequence ofmodulated data symbols, the modulated data symbols having been generatedby invertible randomized spreading of a second stream of data symbols;and second computing means for operating on the sequence of modulateddata symbols to produce an estimate of the second stream of datasymbols.
 35. The transceiver of claim 34 further comprising means toapply diversity to the modulated data symbols before transmission, andmeans to combine received diversity signals.
 36. The transceiver ofclaim 34 in which the second computing means comprises: a correlator forcorrelating each modulated data symbol from the received sequence ofmodulated data symbols with a code from the set of up to M directsequence spread spectrum codes; and a detector for detecting an estimateof the data symbols from output of the correlator.
 37. The transceiverof claim 34 in which the second computing means comprises an inversetransformer for regenerating an estimate of the data symbols.
 38. Thetransceiver of claim 33 further comprising a shaper for shaping thecombined modulated data symbols for transmission.
 39. The transceiver ofclaim 33 further comprising means to apply diversity to the combinedmodulated data symbols before transmission.
 40. The transceiver of claim33 in which the data symbols include a pilot frame and a number of dataframes, and is preceded by a request frame, wherein the request frame isused to wake up receiving transceivers, synchronize reception of thedata symbols and convey protocol information.